1. Field of the Invention
This invention relates to substrates for optical recording media for recording and reproducing information by light, such as laser beam, etc., particularly substrates for optical cards and to a process for preparing the same.
2. Related Background Art
In optical recording media, information is recorded by forming pits in the recording layer by irradiation of laser beam with or without the simultaneous application of an appropriate external energy or force, for example, heat or magnetic force, where the following types are known:
1. Pit-forming type using a recording layer of Te, Te-C, etc., PA1 2. Rim-forming type using an organic pigment, PA1 3. Phase change type by inorganic multicomponent compounds such as Ge-Sn-Te, TeO-SnGe, etc., and PA1 4. Magneto-optical type which records by changing the spin direction by using multi-comonent compounds such as Tb-Fe-Co, Tb-Fe-Gd-Co, etc., and conducting photoirradiation while applying a magnetic field thereto. PA1 .delta.: retardation PA1 .lambda.: wavelength PA1 C : photoelasticity coefficient PA1 .delta..sub.1 -.delta..sub.2 =stress difference
The pits formed in the recording layer by any of these procedures are exposed to a substantially weaker laser beam than the recording beam, and the reflected light from the recording layer is detected to determine the presence or absence of pits and reproduce the recorded information.
In such an optical recording medium, as shown in FIG. 2B, usually a recording layer 62 is formed on a transparent plastic substrate 28, and irradiation of a laser beam or receiving of reflected light for recording or reproduction is carried out through the substrate 28.
Generally, irradiation of a laser beam and receiving of reflected light are carried out together with a polarizing beam splitter (PBS) 22 and a quarter wave plate (QWP) 23 in order to eliminate the return light of the laser beam and efficiently receive the reflected light, as shown in FIG. 2A.
That is, the straight polarized beam emitted from a semiconductor laser 21 passes through PBS 22 and then passes through QWP 23 to be converted to a circular polarized beam, which is focused to a spot, about 1 .mu.m in diameter, through a focusing lens 24 to enter the substrate 28 and irradiate the recording layer 62. Then, the light reflected on the recording layer 62 passes again through QWP 23 to be converted again to the straight polarized beam, which enters PBS 22. At that time, as the reflected light differs by 90.degree. in the polarizing plane from the incident light, the reflected light can not pass PBS, and is reflected by PBS to reach a photo detector (PD) 25.
However, as the reproduction of information is carried out with the reflected light from the recording layer through the substrate, as mentioned above, if the substrate 28 has a refractivity, the reflected light is not converted to the straight polarized beam exactly by 90.degree. difference in the polarizing plane from the incident light when the reflected light passes through QWP, and a portion of the reflected light is not reflected by PBS but passes through PBS, and the not-reflected light returns to the semiconductor laser, causing noise on the light source, and the S/N ratio lowers due to the decrease in the reflected light quantity which reaches PD.
Double refraction of the substrate occurs owing to the photoelasticity coefficient C peculiar to a material multiplied by a residual mechanical stress difference per se, as shown by the following formula (1): EQU Double refraction:(BR)[nm]=(.lambda..delta./2).multidot.C.multidot.t (.delta..sub.1 -.delta..sub.2) (1)
where
As a material for substrate 28 for information recording medium, such as optical disk, etc. polycarbonate is regarded as promising owing to low hydroscopicity, high heat resistance and distinguished moldability. However, polycarbonate resin has so high a photoelasticity coefficient that double refraction is very liable to occur. Thus, in order to suppress double refraction when a substrate for optical disk having preformats such as track grooves or pits is prepared by molding the polycarbonate resin, processes and conditions for molding the polycarbonate to prevent occurrences of residual strains as much as possible, have been investigated, and polycarbonate substrates with low double refraction have been prepared.
An optical card, is an optical recording medium whose recording and reproduction of information are conducted by relative reciprocal movements to a light beam for recording and reproduction. It is preferably bent to a slight degree with rollers 27 to eliminate vibration and slipping of the optical card when the optical card is subjected to reciprocal movement in the direction F traversing with respect to the light source 21 by a driving roller 26 during the recording and reproduction as shown in FIGS. 2A and 2B. However, when the polycarbonate substrate is bent in such a manner as above, a double refraction occurs or increases on the substrate 28, resulting in lowering of S/N ratio of the signal.